The present invention relates to the fabrication of small three dimensional structures, particularly to the fabrication of three dimensional circuit structures used in traveling wave tubes, and most specifically to methods for fabricating helical circuit structures for use in traveling wave tubes.
In traveling wave tubes (TWT""s) an electron beam interacts with a propagating electromagnetic wave to amplify the energy of the electromagnetic wave. To achieve the desired interaction between the electron beam and the electromagnetic wave, the electromagnetic wave is propagated through a structure which slows the axial propagation of the electromagnetic wave and brings it into synchronism with the velocity of the electron beam. In a TWT, one such so-called slow wave is a helical coil that surrounds the structure of the electron beam. The kinetic energy in the electron beam is coupled into the electromagnetic wave, amplifying the wave significantly. The advantages of such slow wave properties in TWT""s are known to those having ordinary skill in the art.
A wide variety of alternative slow wave structures are known. For example, those structures disclosed in U.S. Pat. Nos. 3,670,196, 4,115,721, 4,005,321, 4,229,676, 2,851,630 and 3,972,005. A number of methods for constructing the helixes of these structures are known. Common fabrication techniques include winding or machining. For example, a thin wire or tape of electrically conductive material may be wound around a mandrel and processed to properly shape the helix to the circular configuration of the mandrel. However, the process of winding the helix places stress on the wired tape, creating a helix of limited stability under operating conditions. Additionally, when heated (for example during annealing or during operation), such wound helixes do not have dimensional stability (i.e. helices formed in this manner have a tendency to distort beyond the tolerances required for reliable operation).
Alternatively, a cylindrical helix may be cut into the desired pattern using electron discharge machining. This process does not produce helices of accurate dimensions. However, this process tends to produce helices that are embrittled and subject to cracking.
Although suitable for some purposes, both machining and winding techniques are subject to serious limitations only capable of reliably manufacturing helixes of relatively large dimensions. However, when used in high frequency applications (for example, so-called xe2x80x9cKa-bandxe2x80x9d, xe2x80x9cQ-bandxe2x80x9d, xe2x80x9cV-bandxe2x80x9d, or xe2x80x9cW-bandxe2x80x9d TWT""s) such conventional techniques do not reliably produce the smaller helixes and circuit structures that are needed for these high frequency applications. For example, in a TWT operating in millimeter wavelengths, at frequencies above 20 GHz, conventional techniques produce TWT circuits that suffer noticeably from mechanical distortion effects and thermo-mechanical relaxation. At frequencies near, for example, 50 GHz, the circuit components (including the helix) are so small that conventional manufacturing techniques can produce satisfactory helixes with only with great difficulty and with often unpredictable quality. A typical traveling wave circuit element features a coaxial dielectric support element which is in physical contact with the circuit element. Due to the effects of mechanical distortion or thermo-mechanical relaxation, conventionally constructed circuit elements physically distort and become separated from the dielectric support. This is undesirable. Also, at these frequencies current processes for manufacturing helixes commonly have a very low product yield. An additional limitation to existing methods of manufacturing are the inability to produce certain advantageous non-helical circuit structures. In short, current manufacturing processes produce helices which are plagued with poor tolerances, dimensional inaccuracies, size limitations, circuit unreliability, and insufficient robustness to service the needs of high frequency TWT""s. Additionally, a number of non-helical circuit structures have been proposed by others. The problem with many of these structures is that until now there has been no satisfactory way to construct them for operation at high frequency.
Accordingly, it is the feature of this invention to provide methods and apparatus for constructing small three dimensional circuit structures having precise physical dimensions to narrow tolerances. It is a further feature of the invention to construct structures demonstrating high dimensional stability and robustness. Structures formed in accordance with the present invention also demonstrate improved thermal performance, reduced rf losses, and increases in overall performance efficiency. A particular feature of the present invention to provide a methodology for constructing thermally and dimensionally stable helical circuit elements for use in TWT""s to exacting tolerances at very small dimensions. It is a further feature of the present invention to provide methods of fabricating heretofore unbuildable circuit elements as well as methods for constructing such elements.
The principles of the present invention contemplate methods for constructing thermally and dimensionally stable three-dimensional TWT circuit structures to narrow tolerances and very small sizes by providing a small hollow preform constructed of a desired material. A coating of photoresist material is applied to the preform. The photoresist coating is treated to form a desired pattern in the photoresist coating such that a portion of the outside surface of the preform is exposed and another portion of the outside surface of said preform remains covered with the photoresist pattern. Subsequently, preform material is removed from the exposed portion of said preform leaving the pattern covered portion in place to create a preform having a desired shape. After shaping, the photoresist coating is stripped from said shaped preform, followed by an optional polishing step.
Additionally, the principles of the present invention contemplate an apparatus for forming small three dimensional circuit structures from preforms comprising a means for supporting the preform on its axis, a means for rotating the preform, an exposure source for directing a light beam onto said preform, a means for shifting said exposure source along said preform, and a means for controlling said rotating means, said shifting means, and said exposure source to achieve a predetermined pattern in the preform.
Also, the principles of the present invention as described above contemplate novel three dimensional structures including a very small helix, a ring bar circuit, a very small finned ladder circuit structure, and a very small slotted finned ladder circuit structure as well as traveling wave tubes incorporating these structures.
Other features of the present invention are disclosed or made apparent in the section entitled xe2x80x9cDETAILED DESCRIPTION OF THE INVENTIONxe2x80x9d.